1. Field of the Invention
The present invention relates to a calling number delivery system and, more particularly, a calling number delivery system for delivering calling number information over a control channel without using a communication channel in a radio channel interval between a base station and a plurality of subscriber""s stations at the occasion of delivering calling number information in a system in which an exchange and a plurality of subscriber""s terminals (telephones) are connected over a radio channel.
2. Description of the Related Art
FIG. 6 is a diagram showing a connection sequence in a conventional calling number delivery system. In FIG. 6, calling number delivery (CND) through an analog line (subscriber""s line) is performed by an FSK modulated signal between a first ringing signal and a second ringing signal at the time of reception of an incoming call.
In a system in which an exchange and a subscriber""s terminal are connected by radio, generally, transmission is executed by using a radio channel consisting of a control channel and a communication channel. Upon receipt of an incoming call, the incoming call is notified to the subscriber""s station over the control channel and an analog signal used for communication is converted to a digital signal which is sent over the communication channel.
In case of performing the calling number delivery in the system in which an exchange and a telephone are connected via a radio channel, a base station has to detect the reception of an incoming call, notify it to a subscriber""s station over a control channel, assure a communication channel to send FSK modulated calling number information, and deliver the calling number information over the communication channel to the subscriber""s station.
In case of delivering the calling number information over the communication channel, it is necessary to establish the communication channel for each reception of a call. For example, even when the subscriber is absent, the communication channel is assured, so that effective use of the communication channel is hindered as a result.
It is an object of the invention to achieve effective use of a communication channel by reading FSK modulated calling number information within a base station, sending the information to a plurality of subscriber""s stations over a control channel, and demodulating the FSK modulated signal by the subscriber""s station, without sending the FSK modulated calling number information over the communication channel.
According to the invention, in order to achieve the object, there is provided a calling number delivery system in which a base station connected to an exchange via a first analog line and a plurality of subscriber""s stations connected to telephones via second analog lines are connected via a radio channel, wherein the base station comprises: ringer detecting means for detecting a ringer when a call sent from the exchange via the first analog line is received; calling number information extracting means for detecting calling number information in an FSK modulated signal transmitted via the first analog line and extracting the calling number information from the FSK modulated signal; and information transferring means for transferring the calling number information and ringer information to the subscriber""s station over a control channel in the radio channel, and the subscriber""s station comprising: information receiving means for receiving the ringer information and the calling number information transferred from the base station over the control channel in the radio channel; ringer generating means for extracting the ringer information from the information receiving means and generating a ringer; FSK modulating means for extracting the calling number information from the information receiving means and FSK modulating the calling number information; and selective transmitting means for performing switching between the FSK modulated calling number information and the ringer and selectively transmitting the switched information to the telephone via the second analog line.
The ringer detecting means in the base station is constructed by a ringer detecting circuit, the calling number information extracting means comprises an FSK demodulating circuit for demodulating the FSK modulated signal in an analog signal transmitted from the exchange and an information extracting circuit for reading the calling number information from the demodulated digital data outputted from the FSK demodulating circuit, and the information transferring means is constructed by a control circuit in the base station.
In order to delay the FSK modulated signal until the information extracting circuit starts operating, the FSK modulating circuit is added with signal delaying means including: an A/D converter for converting the FSK modulated signal supplied through the first analog line into a digital signal; a signal delaying circuit for delaying an output of the A/D converter, and a D/A converter for converting an output of the signal delaying circuit into an analog signal and supplying the analog signal to the FSK demodulating circuit.
The information receiving means in the subscriber""s station is constructed by a control circuit in the subscriber""s station, the FSK modulating means is constructed by an FSK modulating circuit, and the selective transmitting means is constructed by a switching circuit.